156 unit 27
a certain type of three-phase power connection that exhibits a higher voltage on one leg when measured to ground than on the other legs.
High leg
It is not uncommon for isolation transformers to be designed with windings that have more than one set of lead wires connected to the primary or secondary. These are called
multiple-tapped windings
The ability of one coil to induce a voltage into another coil is called
mutual induction.
one method of color coding the high leg connections of some three-phase power systems.
orange wire
As current increases from zero to its peak positive point, a magnetic field expands
outward around the coil
Impedance is expressed as a
percent (%Z or %IZ)
In a 60-hertz AC circuit, the voltage changes
polarity 60 times per second
The autotransformer does have one disadvantage. Because the load is connected to one side of the powerline, there is no line isolation between the incoming
power and the load.
The primary winding of a transformer is the
power input winding
The amount of core material needed for a particular transformer is determined by the
power rating of the transformer
This can cause problems with certain types of equipment and must be a consideration when designing a
power system.
The current of the secondary induces a countervoltage in the secondary windings that is in opposition to the countervoltage induced in the
primary
When the primary of a transformer is connected to power but there is no load connected to the secondary, current is limited by the inductive reactance of the
primary
All transformation of voltage and current takes place between the
primary and secondary windings.
An autotransformer has only one winding used for both the
primary and secondary.
As a general rule, connecting the secondary windings of transformers in series does not present a
problem
Loads that are intended to operate on 240 volts, such as water heaters, electric-resistance heating units, and central air conditioners are connected directly across the lines of the
secondary
The magnetic field of the primary induces a voltage into the
secondary
Loads that are intended to operate on 120 volts connect from the center tap, or neutral, to one of the
secondary lines
When a coil induces a voltage into itself, it is known as
self induction
Transformer secondary windings connected in
series
Voltage spikes are generally of very
short duration
a transformer that produces a lower voltage at its secondary than is applied to its primary.
step down transformer
a core made by winding a continuous length of magnetic metal into a spiral.
tape wound core/ toroid core
single-phase transformers connected in such a way as to operate on a three-phase system.
3 phase bank
A blank is a magnetically operated machine that can change values of voltage, current, and impedance without a change of frequency
A transformer
transformers that use only one winding for both primary and secondary.
Auto transformers
a three-phase transformer connection with a delta-connected primary and a wye-connected secondary.
Delta wye
A three-phase transformer connection that can provide a six-phase output.
Diametrical connection
a type of insulating oil often used to cool transformers.
Dielectric oil
When calculating the values of an isolation transformer with multiple secondary windings, each secondary must be treated as a
Different transformer
a method of identifying conductors or components.
Tagging
A MEGGER will reveal problems of insulation breakdown that an ohmmeter will
not
a type of wiring diagram that shows basic power runs and does not show actual connections.
one leg diagram
a type of three-phase transformer connection that employs only two transformers.
open delta
The control transformer is used to reduce the line voltage to the value needed to
operate control circuits.
the winding of a transformer to which the power line is connected.
primary winding
. The most common type of control transformer contains two
primary windings and one secondary
The secondary winding should be disconnected before disconnecting the
primary wing
Street lamps are often connected in
series
a transformer that produces a higher voltage at its secondary than is applied to its primary.
step up transformer
Flux leakage is the amount of magnetic flux lines that do not follow the metal core and are lost to the
surrounding air
Transformer impedance is determined by the physical construction of the
transformer
. Another very important rule is that only phase values of voltage and current can be used when calculating
transformer values
All values of a transformer are proportional to its
turns ratio.
the ratio of the number of turns in the primary winding of a transformer as compared to the turns of wire in the secondary winding.
turns ratio.
Series-connected lamps require a single conductor to be connected from lamp to lamp instead of
two conductors
The neutral carries the sum of the
unbalanced load
If power is applied to the transformer bank before the delta connection is closed, the voltmeter should indicate 0
volts
determined by dividing the voltage applied to the primary of a transformer by the number of turns of wire in the primary.
volts per turn ratio
These dots signify that the polarity is the same at that point in time for each
winding
the winding of a transformer to which a load is connected.
Secondary wing
loads that operate on only one phase of an AC power system.
Single phase load
Blank are the most efficient machines known
Transformer
If the rotary switch shown in Figure 27-41 were to be removed and replaced with a sliding tap that made contact directly to the transformer winding, the turns ratio could be
adjusted continuously.
Magnetic materials contain tiny magnetic structures in their molecular material known as
magnetic domains
Isolation transformer is
magnetically coupled, not electrically coupled
If the phase winding of one transformer is reversed, an extremely high current will flow when power is
applied
The tape-wound core is one of the most efficient core designs
available
The polarity of transformer windings can be determined by connecting them as an autotransformer and testing for additive or subtractive polarity, often referred to as a
boost or buck connection
making the final connection to complete a delta connection.
closing a delta
The primary leads of a control transformer are generally cross
connected
a transformer that supplies a constant amount of current to a varying load.
constant-current transformer
a common type of isolation transformer generally used to change line voltage of 240 or 480 volts into 120 volts to operate motor-control equipment.
control transformer
see constant-current transformer.
current regulator
When lamps are connected in series, some device must be used to continue the circuit in the event that one or
more lamps fail
A wye-delta transformer, for example, has its primary winding connected in a wye and its secondary winding connected in a
delta
Three-phase transformers are connected in
delta or wye configurations.
External gauges are generally provided to indicate the level of
dielectric oil, temperature, and internal pressure
he transformer is shown before it is mounted in an enclosure, which will be filled with a
dielectric oil.
a common transformer used by utility companies to change their line voltage into the 240/120 volts necessary to operate homes and small businesses.
distribution transformer
Constant-current transformers contain primary and secondary windings that are movable with respect to
each other
These dots signify that the polarity is the same at that point in time for
each winding
An isolation transformer has its primary and secondary windings that are
electrically separated from each other.
(1) the DC used to produce electromagnetism in the fields of a DC motor or generator, or in the rotor of an alternator or synchronous motor; (2) the AC that flows in the primary of a transformer with no load connected.
excitation current
DC through an inductor rises at an
exponential rate.
When the current decreases from its peak positive point toward zero, the magnetic field
fields
When closing a delta, connections should be checked for proper polarity before making the
final connection and applying power
The countervoltage of the secondary weakens the countervoltage of the primary and permits more primary current to
flow
When a load is connected to the secondary, current begins to
flow
the number of magnetic flux lines that radiate into space and do not cut through the conductors of an inductor or secondary winding of a transformer.
flux leakage
The H core, however, surrounds the windings on
four sides instead of two
Impedance is expressed as a percent (%Z or %IZ) and is measured by connecting a short circuit across the low-voltage winding of the transformer and then connecting a variable voltage source to the
high-voltage winding
It is the winding that is connected to the
incoming power supply
. Some isolation transformers are built with a turn's ratio of 1:1. A transformer of this type has the same
input and output voltage and is used for the purpose of isolation only.
the amount of current that is drawn when an inductive device is first connected to the power line.
inrush current
something made of thin sheets stacked together.
laminated
A film cutout device shorts and maintains the circuit if the
lamp fails
An inductor maintains the circuit if the
lamp fails
. Constant-current transformers are designed to deliver a constant output current, generally 6.6 amperes, under varying
load conditions.
As a general rule, the low-voltage winding is placed closest to the core and the high-voltage winding is placed over the
low-voltage winding.
The amount of inrush current in the primary of a transformer is limited by three factors:
the amount of applied voltage, the resistance of the wire in the primary winding, and the flux change of the magnetic field in the core. The amount of flux change determines the amount of inductive reactance produced in the primary winding when power is applied.
The open-delta transformer connection can be made with only two transformers instead of
three
If three-phase transformation is needed and a three-phase transformer of the proper size and turns ratio is not available, three single-phase transformers can be connected to form a
three-phase bank.
The isolation transformer greatly reduces the
voltage spike
. All windings should be tested for grounds, and the ohmmeter should indicate infinity for each
winding
The secondary winding is the load
winding, or output winding.
A delta-wye transformer would have its primary winding connected in delta and its secondary connected in
wye
The primary winding is first tied into a
wye connection
a three-phase transformer connection that has its primary connected in wye and a secondary connected in delta.
wye delta